Information Technology Reference
In-Depth Information
Foundation Topics
OSPF
OSPF is defined in RFC 2328. It is a link-state routing protocol that uses Dijkstra's shortest path
first (SPF) algorithm to calculate paths to destinations. In OSPF, each router sends link-state
advertisements about itself and its links to all its adjacent routers. Each router that receives a
link-state advertisement records the information in its topology database and sends a copy of
the link-state advertisement to each of its adjacencies (other than the one that originally sent the
advertisement). All the link-state advertisements reach all routers in an area, which enables each
router in the area to have an identical topology database that describes the routers and links
within that area. The router is not sending routing tables but is sending link-state information
about its interfaces. When the topology databases are complete, each router individually calculates
a loop-free, shortest-path tree to each destination by running the SPF algorithm. The routing
table is built from the shortest-path tree. Destinations outside the area are also advertised in
link-state advertisements. These, however, do not require that routers run the SPF algorithm
before they are added to the routing table.
OSPF is a classless routing protocol that permits the use of variable-length subnet masks (VLSMs).
It also supports equal-cost multipath load balancing and neighbor authentication. OSPF uses
multicast addresses to communicate between routers. OSPF runs over IP protocol 89.
OSPF Concepts and Design
This section covers OSPF theory and design concepts. OSPF link-state advertisements, area
types, and router types are discussed.
OSPF Metric
OSPF uses the cost metric, which is an unsigned 16-bit integer in the range of 1 to 65,535. The
default cost for interfaces is calculated based on the bandwidth in the formula 10
8
/ BW, with
BW being the bandwidth of the interface expressed as a full integer of bps. If the result is
smaller than 1, the cost is set to 1. A 10BaseT (10Mbps = 10
7
bps) interface has a cost of 10
8
/
10
7
= 10. OSPF performs a summation of the costs to reach a destination; the lowest cost is the
preferred path. Some sample interface metrics are shown in Table 8-1.
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